The economic assessment of process control quality using a Taguchi-based method

This paper derives a measure of dynamic control quality within the Taguchi framework (G. Taguchi, E.A. Elsayed, T. Hsiang, Quality Engineering in Production Systems, McGraw-Hill, New York, 1989) which estimates product quality (in economic terms) by the losses incurred when specified product characteristics depart from their nominal values. In this sense, a control system may be viewed as a “product” with stability and performance properties as its required characteristics. The degree to which system inputs and outputs depart from their nominal values is used to give an estimate of stability and performance quality, respectively. Displaying control quality graphically with stability and performance qualities as the two axes can be used to highlight the trade-off between these two characteristics. This technique was used to evaluate the quality offered by four model-based controllers on a distillation column model responding to a typical disturbance, and how the presence of measurement and valve dynamics affected control quality. It is anticipated that the measures derived in this paper (being both time-based and economic) may prove useful in conveying control quality in a common language understood by engineers, operators and management